Angstrom-thick or thicker layers of metals, metal compounds, and other agents have conventionally been used in multiple layer glass panels to reflect heat-producing infra-red solar radiation while transmitting visible light. These layers can be arranged in sequence as stacks, and can be disposed on any appropriate substrate, such as biaxially stretched, thermoplastic poly(ethylene terephthalate) (PET) film or similar material. One form, known as an interference filter, comprises at least one layer of reflective metal sandwiched between reflection-suppressing or anti-reflective dielectric layers.
When a metallized film such as an interference filter is combined with glass in a multiple layer glass laminate, for example in a vehicle windshield, a shock-dissipating interlayer of plasticized poly(vinyl butyral) (PVB) is usually included to absorb a blow from the head of an occupant within the vehicle or from a foreign object from outside the vehicle without allowing penetration through the windshield. In a typical arrangement, a single layer of a poly(ethylene terephthalate)-type coated film is disposed between two layers of a poly(vinyl butyral)-type material to form a three layer structure that is subsequently disposed between two layers of glass. The finished multiple layer glass panel has the combined benefit of safety and radiation transmission control.
An optical quality defect that can be particularly noticeable at oblique viewing angles can occur in such safety glass panels as a visually apparent, isotropic, wave-like reflected image estimated to have an amplitude of about 0.002 to 0.012 mm and a wavelength of 2.5-7.5 mm. It is referred to hereinafter by the term “applesauce.” Applesauce is believed to occur because the polyethylene-type layer, which reflects light at oblique viewing angles, conforms to the poly(vinyl butyral)-type layer during lamination and will assume any non-linearity, or waviness, that is present on that poly(vinyl butyral)-type layer. Other objectionable visual defects, such as “mottle” (mottled color appearance), can also occur in colored poly(vinyl butyral) glass laminates when poly(vinyl butyral)-type layers are disposed in contact with each other. For example, when pigmented poly(vinyl butyral) is coextruded with non-pigmented poly(vinyl butyral) and the resulting layer is then laminated with other layers or glass, mottle can result.
Several attempts to mitigate the severity of applesauce and mottle have been reported (see, for example, U.S. Pat. Nos. 4,465,736, 4,973,511, and 5,091,258), however, further methods of reducing applesauce and mottle are needed. Accordingly, further improved compositions and methods are needed to enhance the characteristics of polymer and poly(vinyl butyral) sheets that are used in conjunction with a polymer film layer, and specifically poly(ethylene terephthalate), without adversely affecting the optical characteristics of the resulting multiple layer structure.